502 Major-G-eneral J. Waterliouse. On the Sensitiveness 



various strengths of hydrogen peroxide sohition, have given fairly 

 sensitive fihns, sometimes a little white in appearance, but as a rule 

 the printed-out images produced on exposure to light are just the 

 reverse of those produced on plain plates and those fumed, as noted 

 above, i.e., the parts exposed appear darker than the unexposed parts 

 and do not attract mercur}^ vapour, so that a positive pictiu-e is pro- 

 duced from an ordinary black and white negative. By long exposure 

 this 'effect is sometimes reversed. The visible images produced on 

 silvered glass plates fumed with the peroxide quickly fade away, 

 though the details may still remain visible on breathing. The same 

 effect of fading out has also been observed on plain silver plates, most 

 markedly on the thin foils. 



Rain or BoiUd Distilled' WoAer. — Silver plates exposed imder fresh, 

 clean rain water, or boiled distilled water, gave distinct images on 

 development with mercury, but not very readily, and further observa- 

 tions are necessary. 



Exposures in Fluid Hydrocarbons. — In order to ascertain whether the 

 images would be produced on silver surfaces exposed in fluid hydro- 

 carbons containing no oxygen, silvered glass and pure silver foil were 

 exposed in fluid paraffin, benzene, xylene, and toluene. 



Paro.ffin. — In clear, colourless, fluid paraffin very strong dark olive- 

 yellow images were produced by light on silvered glass and silver foil 

 on the parts exposed freely in the fluid or under the parts cut out of a 

 black cardboard mask but under a mica screen, which covered part of 

 the black mask, and of the plate itself, there was no darkening action 

 whatever. The darkening appears to be due to sulphuration, but why 

 it should not occui' under the mica is not clear, unless the latter cuts 

 off the rays which are active in producing it. 



Benzene. — Silver foil well cleaned and exposed in ordinary rectified 

 benzene under a cut-out black paper mask, and also half covered with 

 mica, showed very clear, yellow images of the cut-out design on the 

 part uncovered by the mica, apparently by decomposition of thio- 

 phene or other sulphur compound present in the benzene. This 

 darkening required strong sunshine to produce it, and on prolonging 

 the exposure in the sun the metal became quite bronzed in the un- 

 covered exposed part, while the part protected by the mica also took a 

 light tint. It may be noted that, as a rule, the backs of these slips of 

 foil (which were exposed in test-tubes) were not protected by any 

 covering, but showed no perceptible change of colour : the darkening 

 of the exposed foil was therefore solely an effect of strong light. 



Effect of Heating the Foil. — A piece of the same foil was exposed in 

 the same way in the same benzene after being heated to redness. 

 This did not show such a strong image, even by long exposure in 

 the sun. 



A piece of piue foil (assay foil) was exposed in a purer sample of 



